Light-water nuclear reactors, and especially pressurized-water nuclear reactors, comprise a vessel of overall cylindrical shape closed by dished bottoms. The cylindrical part consists of forged shells which are welded end to end at the time when the vessel is being assembled. One of these shells, called a pipe-carrying shell, comprises openings passing through its wall in the region of which is produced the fastening of pipes intended to be connected to the pipework forming the hot legs and the cold legs of the primary circuit loops.
The process employed most widely until now for producing the fastening of the pipes to the pipe-carrying shell consists in providing openings passing through the shell of a diameter greater than the external diameter of the pipe which is engaged in the opening over the entire thickness of the shell wall. Parts which are machined in a corresponding manner on the pipe and in the opening of the shell permit one or more welding chamfers to be produced, which are then filled with filler metal from the outside and/or the inside of the shell. This operation is carried out using the automatic submerged arc welding process.
In the case of nuclear reactors whose vessel has a diameter of the order of 4.50 meters, the thickness of the pipe-carrying shell is slightly below 300 mm, this thickness being substantially uniform in all parts of the shell.
A different process for fastening the pipes of a pipe-carrying shell has also been proposed. This process consists in providing in the pipe-carrying shell openings whose diameter corresponds substantially to the internal diameter of the pipe, which is welded over its entire thickness in the extension of the opening, on the outer side of the shell wall.
This method of fastening the pipe, where the latter is added, i.e., "set" on the outer surface of the shell, is referred to by the expression "set-on", in contrast to the method of fastening described above, where the tube enters the opening passing through the shell, this method of fastening being referred to by the expression "set-in".
In theory, the "set-on" configuration resulting from the second method of fastening offers advantages over the "set-in" configuration, since a weld fault giving rise to a failure would constitute an accident of a type provided for in the safety specifications of nuclear power stations, this accident being equivalent to the failure of a primary circuit leg, resulting in a loss of coolant at a very high rate.
In the case of the "set-in" configuration, a fault in the weld of a pipe resulting in a failure constitutes an accident equivalent to a vessel failure.
However, the "set-on" design requires the use of a pipe-carrying shell of increased thickness and the presence of outwardly projecting parts on this shell, onto which the pipes are set. In fact, it is not possible to carry out automatic submerged arc welding of the pipe in a region which is flush with the external surface of the shell, because of the bulk of the welding head.
In the case of a reactor vessel with a diameter close to 4.50 m, it is therefore necessary to provide a pipe-carrying shell whose thickness is of the order of 400 mm in its main part and which can go up to 450 mm in the region of the projections onto which the pipes are set. The forging of such shells requires the use of initial ingots of very great mass, of the order of 350 tons.
Furthermore, the execution of automatic submerged arc welding of the pipes is a tricky operation which requires numerous checks.
More generally, in the case of reactors or of vessels of any type whatever which comprise a thick wall, for example thicker than 100 mm, there was no known process for fastening pipes by welding, simple to implement and requiring a shorter time to perform, enabling a very high weld quality to be ensured.